1. Field of the Invention
The present invention relates to a lens having a diaphragm for converging a collimated light beam and an optical unit which utilizes an optical system using the lens and using a collimated light beam. For example, the invention is concerned with a lens suitable for use in a magneto-optic disk unit for recording and reproducing information to and from a magneto-optic disk or in an optical communication system.
2. Description of the Prior Art
A conventional optical unit which utilizes an optical system using a collimated light beam will now be described with reference to an optical communication system as an example. An optical communication system is known in which a signal is subjected to photoelectric conversion, a corresponding light beam is emitted from a laser diode and is collimated by a collimator lens, and the thus-collimated beam is converged on an end face of an optical fiber. As another conventional optical unit there is known an optical head unit which performs information recording and reproducing operations for a magneto-optic disk (a recording medium) and in which a diaphragm mechanism for restricting the beam diameter is attached to an objective lens which is for converging a laser beam as a beam spot onto a recording/reproducing side of the recording medium.
Now, with reference to FIG. 5, the following description is provided about a lens having a diaphragm structure in a conventional optical unit.
FIG. 5 is a sectional view showing a conventional lens. As shown in the same figure, a lens 11 (for example an objective lens) is a circular convex lens obtained by molding a transparent optical material capable of being molded such as glass or plastic material.
The lens 11 is provided with a central incident lens portion 12 projecting outward (upward) in a generally semispherical shape, an exit lens portion 13 projecting in a generally circular shape outward (downward) on the side opposite to the incident lens portion 12, and an annular flange portion 14 provided along outer edges of the incident lens portion 12 and the exit lens portion 13.
The flange portion 14 has an incident surface 14a located on the incident lens portion 12 side and an exit surface 14b located on the exit lens portion 13 side. The incident surface 14a and the exit surface 14b are parallel to each other and perpendicular to an optical axis of the lens 11.
The flange portion 14 thus formed perpendicularly to the optical axis of the lens 11 exhibits a diaphragm function for the lens 11.
The operation of the conventional lens will be described below on the basis of the optical unit.
An incident light beam A (laser beam) is applied at a predetermined beam diameter and in parallel with the optical axis of the lens 11 from a laser beam source (not shown) disposed on the incident lens portion 12 side of the lens 11 (for example an objective lens). A portion of the parallel incident beam A incident on the incident lens portion 12 is converted as a beam spot by both incident and exit lens portions 12, 13. The beam spot is focused on a recording medium (not shown) and is reflected by the recording medium. The reflected beam passes through the lens 11 and is detected by a photodetector (not shown) disposed on the incident lens portion 12 side.
On the other hand, a portion of the parallel incident beam A is incident on the flange portion 14 and is partially reflected by the incident surface 14a of the flange portion. The thus-reflected incident beam portion returns in parallel with the optical axis of the lens 11, that is, along the incident optical axis. A portion of the incident beam A passes through the incident surface 14a and is not converged as a beam spot. Thus, the flange portion 14 functions as a diaphragm mechanism.
In the conventional lens 11, as described above, since the diaphragm mechanism is constituted of the incident surface 14a of the flange portion 14 which is formed so as to intersect the incident optical axis perpendicularly, a portion of the beam incident on the incident surface 14a is reflected by the same surface and returns to the incident beam side. This undesirable reflected beam joins the information-carrying return beam reflected from the recording medium and is converged on the light receiving surface of the photodetector. Once the reflected beam returned from the incident surface 14a is detected by the photodetector, there arises noise and thus the return beam from the recording medium can no longer be detected with a high accuracy, with consequent occurrence of a detection error.
In an optical communication system using the conventional lens, the undesirable beam reflected by the incident surface 14a as a diaphragm portion becomes a return beam to the laser diode, giving rise to the problem that there occurs an output variation or a wavelength variation of the laser beam.
In a lens having a diaphragm structure, this problem may be solved by forming a reflection preventing film on an outer edge portion of the lens or by roughening (matting) an incident surface to scatter an incident light beam. However, for forming a reflection preventing film or a matted surface for the flange portion of the lens, it is necessary to provide separate such film or surface forming steps for the flange portion and an effective lens portion (masking is required in an alternate manner) or it is necessary to hold a side face of the lens flange portion with a jig.
No matter which method may be adopted for the formation of such reflection preventing film or matted surface, an increase of the manufacturing cost is unavoidable due to an increase in the number of steps or due to the provision of a special jig.
Besides, it is impossible to form the flange portion small because an allowance for machining is required in forming the reflection preventing film or matted surface. Consequently, the lens becomes larger in diameter and it is therefore impossible to reduce the size and weight of the optical unit.
The present invention has been accomplished for solving the above-mentioned problems and it is an object of the invention to provide a lens with a diaphragm capable of being fabricated easily and capable of diminishing the influence of an undesirable return light, as well as an optical unit using the lens which optical unit can be reduced in size and weight.
According to the present invention there is provided a lens for converging a collimated incident light and outputting the resulting converged light, the lens having a diaphragm structure at an outer edge portion of an incident side thereof, said diaphragm structure being constructed in such a manner that a portion of the collimated incident light is reflected and the reflected light advances away from or toward an optical axis of the collimated light.
In the lens of the present invention, the diaphragm structure has a collimated light incident surface which is an arcuate surface or an inclined surface.
According to the present invention there also is provided an optical unit using a lens for converging a collimated incident light and outputting the resulting converged light, the lens having a diaphragm structure at an outer edge portion of an incident side thereof, the diaphragm structure being constructed in such manner that a portion of the collimated incident light is reflected and the reflected light advances away from or toward an optical axis of the collimated light.